[Bug target/53383] Allow -mpreferred-stack-boundary=3 on x86-64

[Jan Hubicka]

> > This isn't necessary.  If __int128 is put on stack by caller,
> > the stack must be aligned at 16 bytes.
> 
> Not when you call function with -fpreferred-stack-boundary=3
> and it is itself compiled with -fpreferred-stack-boudnary=4
> and calls another functions passing __int128.
> Thus the ABI incompatibility.
... and the alignment code is there not to align stack frame, but to
align the __int128bit position within the argument area.  The
va_arg calls beforehand are of unknon count.
> Honza